Sandalwood oil is an essential oil obtained from sandalwood tree occurring in the east of India and is highly valued as a perfume component. However, production of sandalwood oil is being decreased because, felling of the sandalwood tree is restricted from the standpoint of environmental and resources conservation pressures. Therefore synthetic sandalwood oil fragrance materials have been developed for substitution.
It is known that the main components responsible for the odor of natural sandalwood oil are .alpha.-santalol and .beta.-santalol. Santalol is difficult to synthesize in quantities because of its chemical structure. In recent years, substances different from santalol in chemical structure but very similar in odor to santalol have been synthesized.
Among such substances are 2-alkyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol derivatives (see JP-B-56-361676 and JP-A-55-139330, the term "JP-B" as used herein means an "examined Japanese patent publication", and the term "JP-A" as used herein means an "unexamined published Japanese patent application"). 2-Methyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol is available from Taiyo Koryo K.K. under the trade name of "Santalinol", and 2-ethyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol from International Flavor and Fragrance (hereinafter abbreviated as IFF) under the registered trademark "Bacdanol".
There is found no report on synthesis of an optically active 2-ethyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol.
R. Reinhardt, et al. have reported in J. Chromatog. A, Vol. 697, No. 7, pp. 475-484 (1995) that the resolution of 2-ethyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol was not successful by gas chromatography using a chiral column.
Thus, there is no report on the isolation or synthesis of an (E)-(R)-2-alkyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol, i.e., an optically active compound of a 2-alkyl-4-(2,2,3-trimethylcyclopent-3-en-1-yl)-2-buten-1-ol in which the trimethylcyclopentene ring and the hydroxymethyl group are in a trans configuration.
Several reports are found with respect to the relation between the configurations of the side chain of fragrance substances having a trimethylcyclopentene ring and the odor.
For example, (1) concerning the relation between odor and the absolute configuration of the asymmetric carbon atom on the 1-position of the cyclopentene ring in compounds having a (2,2,3-trimethyl-3-cyclopenten-1-yl)alkanol skeleton, U.S. Pat. No. 4,052,341 and U.S. Pat. No. 4,610,813 have reported that the (R)-isomer and the (S)-isomer have the same odor as the racemate, that is, the absolute configuration has no influence on odor.
(2) As for 3-methyl-5-(2,2,3-trimethyl-3-cyclopenten-1-yl)-4-penten-2-ol, U.S. Pat. No. 4,696,766 has disclosed that the relationship of the odor and relative configuration of 2 and 3-positions that a mixture of anti-diastereomers is 500 times that of a syn-form diastereomer mixture, but has no mention of the relation between the absolute configuration of the 1-position of the trimethylcyclopentene ring and the odor.
(3) Regarding the four optical isomers and diastereomer mixtures of (E)-3,3-dimethyl-5-(2,2,3-trimethylcyclopent-3-en-1-yl)-4-penten-2-ol, U.S. Pat. No. 5,512,543 has described that the (1R)-isomer (mixture of diastereomers in which the 2-position is racemic) has a milky and strong sandalwood-like odor; the (1S)-isomer (mixture of diastereomers in which the 2-position is racemic) is drier than the (1R)-isomer and has a sandal cedar note; the (1S,2S)-isomer has a milky and strong sandalwood oil odor with a faint animal note; the (1S,2R)-isomer has a weak and flat sandalwood-like odor; the (1R,2S)-isomer has a more pleasant odor than the (1R,2R)-isomer and bears the odor of the (1R)-isomer (diastereomer mixture in which the 2-position is racemic); and a (2S)-isomer (mixture of diastereomers in which the 1-position is racemic) has a pleasant odor.